1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to improved techniques for manufacturing birefringent optical fiber.
2. Background Art
In the field of fiber optics, a polarization-maintaining fiber (PM fiber) is a type of optical fiber in which the polarization of linearly-polarized light waves launched into the fiber is maintained during propagation. One common type of PM fiber is the PANDA (“Polarization-maintaining AND Absorption-reducing”) design, in which a high degree of fiber birefringence is created by adding stress rods to the cladding region of a fiber preform. PANDA-style fibers were originally developed for the telecommunications industry. Over the years, the use of these fibers has expanded to a number of other applications, including fiber optic gyros, fiber optic sensors, and high-power fiber lasers.
Currently, PANDA-style fibers are manufactured using a costly, painstaking technique. A preform rod is fabricated. A skilled operator uses special equipment to drill precisely positioned first and second holes down the length of the rod, parallel to the rod axis. Respective first and second stress rods are fitted into the first and second holes. The stress rods are chemically doped to have a different degree of thermal expansion than that of the material used to fabricate the preform rod. When a fiber is drawn from a preform incorporating the preform rod, the stress rods introduce mechanical stress with a well-defined orientation.
The described technique suffers from a number of disadvantages. First, the hole-drilling operation is difficult to perform and is typically done by specialized glass working companies. Second, the amount of fiber yielded by a preform depends upon its length. In the case of a PANDA-style fiber, the length of the preform is limited by the maximum drilling depth of the precision drilling rig used to drill the holes into the preform rod.